SOI complementary metal-oxide-semiconductor (CMOS) transistors have many advantages over conventional transistors such as bulk CMOS transistors. An SOI transistor suffers, however, from one inherent flaw. The floating body of the SOI transistor can develop a body charge over time. The amount of such body charge will depend on the potentials at the source, drain and gate of the SOI transistor. Generally, the highest amount of charging will occur when the gate is off and both the source and drain are biased at the same high potential. Given enough time and/or potential at the source and drain, the body charge of the SOI transistor will eventually reach a saturation level.
The accumulation of body charge on an SOI transistor creates a problem. During subsequent switching of the source or drain of the SOI transistor, the body charge will eventually be discharged by means of a transient bipolar current. This discharge, in turn, can create a number of additional problems, depending on the application in which the SOI transistor is used. First, a discharge of transient bipolar current can cause speed degradation for an initial cycle in some types of SOI circuits such as wide multiplexer configurations (e.g., long array bitlines, multiplexers with virtual grounds, OR/NOR gates). Second, such discharge can cause functionality errors depending on the amount of time required to remove the body charge. This time will generally depend on the size and number of devices serving as parallel discharge paths to ground. For example, the errors are generally more pronounced for nFETs (n-field effect transistors) serving as paths to ground when the common sources and drains, as well as the nFET bodies themselves, are initially charged to a high potential and subsequently discharged at the common node. This is due to the inherent higher current gains of a parasitic npn transistor. Following an initial cycle discharge, this effect will be seen again if the SOI transistor bodies have enough time to charge again. This body charging behavior, which saturates on the order of milliseconds, is unique to SOI devices. The added uncertainty of initial conditions in SOI circuits caused by the body charging, makes the circuit behavior dependent on duty cycle, a problem not seen in conventional bulk CMOS circuits.